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#2551 07/12/01 09:19 PM
Joined: Nov 2000
Posts: 123
D
doc Offline OP
Member
here is the setup you have several ac units all 480 volt 3 phase each unit pulls 22 amps, the runs are over 100 ft. we are running inch and a half 1 1/2" EMT inside that we are running 36 wires all #10 thhn now I am being told that I only have to derate my wires to 80 per cent?
Would you guys run this many wires in there and is this correct for derating ,I think it needs to be dropped to 40 per cent according to the derating table ,but these guys are trying to tell me that is for heating purposes only Ihave been looking at tarticle 310 on adjustment factors any feed back or am i ok at using 80 per cent


MAY THE SUN SHINE ON YOUR FACE IN THE MORNING AND YOU AWAKE WITH A SMILE
Joined: Nov 2000
Posts: 2,148
R
Member
Doc,
If the units have an actual load of 22 amps the ampacity of the wire must be 27.5 amps. The ampacity of a #10 THHN in a conduit with 35 other current carrying conductors is only 16 amps. You must apply Table 310-15(b)(2)(a). The required wire size is #6 if you place all 36 conductors in the same raceway.
Don(resqcapt19)


Don(resqcapt19)
Joined: Jul 2001
Posts: 62
J
Member
id run 1 conduit for every 2 units,i hope your not puting your stat wires in the same pipe also.remember ac units can run for long periods of time ...i would run #8's to feed them

Joined: Nov 2000
Posts: 123
D
doc Offline OP
Member
Thanks for the confirmation guys I have been arguing this for 2 days with my boss but due to him hiring a electric contractor and we have a journeyman on staff and they both say 310 is for heat only and since # 10 is really good for 40 amps they can still run all that wire and put the 22 amp load on it,what is sad in this whole situation is that 2 young guys are torn between believing me or the 2 guys with the liscense .As for the T-stat wires it would not surprise me if they do run them in it.I never try to argue the code but sometimes you have to so I will go at my boss again tomorrow and show him this,sure glad he is a good guy


MAY THE SUN SHINE ON YOUR FACE IN THE MORNING AND YOU AWAKE WITH A SMILE
Joined: Nov 2000
Posts: 2,148
R
Member
doc,
The required derating in Table 310-15(b)(2)(a) is for heat. Every current carrying conductor produces heat and the more you put in the same raceway the more heat you have. When you limit the current that is permitted to flow in each conductor you reduce the heat produced. In many cases you also have to apply the correction factors for high ambient temperature at the bottom of Table 310-16 in addition to the derating required by 310-15(b)(2)(a).
Don


Don(resqcapt19)
Joined: Jan 2001
Posts: 1,044
Tom Offline
Member
Doc,

A quick look at table 310-15(b)(2)(a) shows that there is a serious derating penaly when you hit the 10th current carrying conductor.

Besides that, it is a serious pain in the butt to deal with that many conductors in one conduit.

Just think, nine #10's each in 4 runs of 3/4" EMT. Lot easier to bend, pull wire, etc and probably cheaper (material) than the 1 1/2" conduit.

Tom


Few things are harder to put up with than the annoyance of a good example.
Joined: Nov 2000
Posts: 123
D
doc Offline OP
Member
Thanks again for confirming what I have been telling these guys along with my boss,Don I even threw in the fact that there is a correction factor involved because a lot of this wiring is on top of the building where the sun is hitting it. Tom that is what I suggested in the first place which got us at ends with each other,if one wire in all of that is bad it can melt the whole bundle and boy when I said that the arguing started.NOW HERE IS a dumber move at every 4 ft roughly they are now tapping those 36 wires together. Am I the only one that works with people with a lisc that does this type of work ? don't understand how a person can just throw his knowledge away and do shoddy work


MAY THE SUN SHINE ON YOUR FACE IN THE MORNING AND YOU AWAKE WITH A SMILE
Joined: Jun 2001
Posts: 29
R
Member
Besides derating, just for ease of use we always went for the next up on conduit size. Thhn always wants to twist depending on the length of the run. It just makes life much easier on the electrician with the the next size and the cost dif is worth it to me. Just passing on an idea.

This was a long time ago but we ran 4" conduit to a magnetic mixer in a steel mill, used a 10 ton crane to pull it around 40'. I know crazy, we used max NEC to fill the conduit and we knew we would break some conductors. I just don't like going by the recommended every since then. Even 1/2" maxed is a real pain.

Joined: Oct 2000
Posts: 5,389
S
Member
I must agree with Resistor on the max fill.

[Linked Image]

also, if you tape 'em together they will snarl up instead of pulling past each other.

Joined: Jun 2001
Posts: 29
R
Member
Sparky, we had to tape them in groups, and taper them, and added soap. It was still a mess and the crane was lifting some of the foundation. Yikes! We never tried that again.

BTW, these were control wires.

[This message has been edited by Resistor (edited 07-14-2001).]

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